The problem with simplex operators is that they cannot hear that they are at a disadvantage when compared to duplex operators.
E.g. I heard a QSO where I think 4 operators were in a roundtable, and two of them were operating simplex.
Due to the delay (and therefore uncertainty that they transmit on a clear channel) they were constantly doubling with others, and the duplex stations had to backoff all the time (of course they could hear when they are doubling).
This is also apparent when calling to "rare stations" on the satellite. Those simplex stations are all transmitting on top of eachother where a duplex station can hear that it is useless to continue shouting.
I think it is part of the personality of some people to not bother about such things.
It is likely inherited from the (also simplex) world of DX'ing where it only counts to make that "you are 59" QSO. And some people just want to have the microphone all the time and do not want to be bothered with others who want to say something too.
However, those that want to listen to others and want to open themselves up to experiences made by others quickly find the advantages of having duplex capability.
Alas, not all people are like that.